The overall objective of our research is the characterization of the inorganic anion permeability pathways of the Ehrlich mouse ascites carcinoma cell. The kinetic characteristics of chloride, sulfate, and inorganic phosphate transport across the plasma membrane suggest that in order for these anions to enter the cell they must first interact with specific membrane carrier systems. Previous studies have provided evidence consistent with the view that chloride and sulfate utilize a single carrier system possessing two reactive sites, while phosphate utilizes an entirely separate system. Recent studies have shown that chloride utilizes three separate permeability pathways: nonmediated diffusion (10%), co-transport with potassium and sodium (40%), and mediated self-exchange through the anion transporter (50%). During the past year, we have investigated the co-transport pathway in some detail. Our results show that in order for chloride to be transported by this mechanism both potassium and sodium are required. In cells under steady state (physiological) conditions, the anion/cation stoichiometry at 37~C is: 2Cl:1Na:1K. Chloride transport mediated by the anion exchanger (Cl self-exchange) exhibits self-inhibition at high extracellular chloride which is abolished when pHo is lowered to 5.5. This finding suggests that H[unreadable]+[unreadable] interacts at a modifier (inhibitory) site on the anion transporter and in so doing relieves inhibition. In contrast to chloride, about 85% of phosphate transport depends on the presence of sodium in the extracellular medium. We have shown that monovalent phosphate is the predominate, if not exclusive, species transported and that a decrease in intracellular pH results in inhibition of transport. In the next year of this project, we plan to further characterize the cation requirements of the chloride-cation co-transport pathway. We are particularly interested in the mechanism by which this transport system switches from a bi-directional exchange during the physiological steady state (constant cell volume) to a net transport pathway when the tumor cell volume regulates. (A)